200 CHEMICAL ENGINEERING the hammers swing freely, there is no danger of breaking the hammer by this operation. The under part of the machine, concentric with the shaft, consists of a grid of steel bars, the distance between them being equal to the desired product. If the blow of the hammers on the rock does not divide it small enough to pass through these apertures, the hammers continue to give the rock additional blows while it is on the grid, the eventual result being that all of it passes through these apertures. Machines of this type are satisfactory for use with comparatively soft rocks such as shales, limestones, clays, etc., or on rock which does not contain an excessively high percentage of silica. These machines will take pieces up to 4, 5, or 6 in. in diameter and the economical fine limit is about Y^ in. It is practical to secure J4 in. product on very soft material, but by diminishing the output size, the capacity of the machine is seriously reduced also. Under proper operating conditions and in fields to which the machine is adaptable, efficient results are secured. The power required is not high, but depends largely upon the character of the rock broken and the ratio of reduction. The accompanying drawing shows the general make-up of a machine of this type. ^ ', „ T fl FIG. 5.—Crushing rolls. The crushing roll is a type of machine that has been in use for a great many years. It consists of two solid cylinders with bearings in the same horizontal plane, set fairly close together, the stream of rock to be crushed passing between the two. One or both of these cylinders may be positively driven. They are usually adjustable so that the rock may be reduced to a desired size. Rolls are usually made with a solid cylindrical core having removable shells which take up wear. These shells are made of hard steel, often so alloyed as to present a surface highly resistive to abrasion. The most favorable condition for crushing rolls is when the ratio of size of feed product does not exceed about 4 or 4J£ : 1. When this ratio is exceeded, the efficiency of the roll is greatly decreased and the power requirement increased. With the ratio of 4 : 1, the power is practically constant whether the ratio is applied to large feed and large product, or fine feed and finer product. The capacity of the machine, however, will usually be in proportion to the size, that is, with production on large pieces, the capacity will be greater than with reduction on smaller pieces; other factors remaining constant.